The invention relates generally to metal oxide non-linear voltage-variable resistors, and is particularly applicable to, but not limited to, zinc oxide resistors for overvoltage surge protection devices.
Various overvoltage surge protective devices include as an essential element a non-linear voltage-variable resistor. The resistor may be made of silicon carbide, which is commonly used for such a purpose, or it may also be made of a metal oxide composition, such as zinc oxide. Overvoltage surge protection devices and their function are discussed in, for example, the following:
U.s. pat. Nos. PA0 Technical Publications PA0 U.s. pat. Nos. PA0 Technical Publications
3,671,800 issued 20 June 1972 to E. W. Stetson PA1 3,586,913 issued 22 June 1971 to A. A. Olsen et al. PA1 2,529,144 issued 7 Nov. 1950 to E. A. Evans et al. PA1 "Electrical Transmission & Distribution Reference Book" 4th Edition, Westinghouse Electric Corporation, Pittsburgh, Pa. 1950 pp. 621 to 627. PA1 3,689,863 issued 5 Sept. 1972 to Matsuoka et al. PA1 3,670,216 issued 13 June 1972 to Masuyama et al. PA1 3,670,221 issued 13 June 1972 to K. Hamamoto et al. PA1 3,663,458 issued 16 May 1972 to Masuyama et al. PA1 3,642,664 issued 15 Feb. 1972 to T. Masuyama et al. PA1 3,634,337 issued 11 Jan. 1972 to M. Matsuoka et al. PA1 3,632,529 issued 4 Jan. 1972 to M. Matsuoka et al. PA1 3,632,528 issued 4 Jan. 1972 to M. Matsuoka et al. PA1 3,611,073 issued 5 Oct. 1971 to K. Hamamoto et al. PA1 3,598,763 issued 10 Aug. 1971 to M. Matsuoka et al. PA1 3,570,002 issued 24 Apr. 1968 to T. Masuyama et al. PA1 3,503,029 issued 24 Mar. 1970 to M. Matsuoka PA1 3,496,512 issued 17 Feb. 1970 to M. Matsuoka et al. PA1 Ichonose, Noboru "TNR High Performance Ceramic Varistor Element", in Japan Electronic Engineering, Tokyo Shibaura Electric Company, Ltd, July 1972 pp. 32-36. PA1 "Metal Oxide Varistors For Power-Surge Protection", in Electrical World, Vol. 177, Feb. 1, 1972 p. 109. PA1 "Unique Variable Resistor Bypasses High Voltage Spikes" in Product Engineering, Vol. 43, Feb. 1972 p. 40. PA1 "Defeating Power Surges", in Research & Development Review 1972 by General Electric Company, Corporate Research & Development, Schnectady, New York, pp. 6-7.
Metal oxide compositions for non-linear resistors are described in some detail in, for example, the following:
While zinc oxide resistors consist substantially of zinc oxide, certain impurities added to the zinc oxide in minute quantities are necessary to give the resistor the characteristics which are desired for a given application. These characteristics are termed the strength, the exponent, and the stability.
The strength of surge arrester valve element is a measure of its ability to resist current channeling in the bulk material under severe loading conditions. The phenomenon by which such channeling occurs is not presently fully understood, and the strength of a given valve element material is thus determined on a relative basis by empirical methods.
The exponent of a valve element material is, in effect, the degree of non-linearity of the resistance relative to the applied voltage. Thus, a high exponent material is more likely to be suitable as surge arrester valve than is a low exponent material, as it will provide a more distinct switching operation.
Stability refers to the ability of the valve element to retain its initial current-voltage characteristics after a period of operation under typical operating conditions.
One composition of zinc oxide which has shown considerable promise as a surge arrester valve material has added to it, in addition to a number of other impurities, some silicon dioxide. The silicon dioxide increases the exponent and the resistance of the material. However, the composition does not have the stability needed to make it truly successful commercially for high voltage surge arrester applications. Even under normal operating conditions, its electrical characteristics change after a relatively short time. Such a change can, in the case of a high voltage surge arrester, result in a catastrophic failure causing extensive damage to other components in the system, which the arrester is designed to protect.